It is known to releasably couple a tool such as a probe or cutting tool (e.g. a boring bar) to a shank, which supports the tool in a machine tool. This enables replacement of a faulty, worn, or broken boring bar for example without replacement of the shank as well, and thus is more cost effective. However, releasable coupling mechanisms have the disadvantage that, during coupling the axis of the tool may be displaced by a small distance from the axis of the shank as a result of lateral forces in the coupling mechanism. This source of error is undesirable, and particularly so where the device to be coupled to the machine is a touch or other probe.